In recent years, a secondary battery, which can be repeatedly charged and discharged, has been widely used as an energy source for wireless mobile devices. In addition, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (Plug-In HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle or large-sized devices, such as vehicles, use a battery module or a battery pack having a plurality of battery cells electrically connected to each other because high output and large capacity are necessary for the middle or large-sized devices. For such a secondary battery, a plurality of battery cells may be connected in series to each other in order to provide output and capacity required by an apparatus or device in which the secondary battery is mounted.
Meanwhile, a lithium secondary battery exhibits excellent electrical properties; however, the lithium secondary battery has low safety. For example, in the event of abnormal operations, such as overcharge, overdischarge, exposure to high temperature, and electrical short circuit, of the lithium secondary battery, decomposition of active materials and an electrolyte, which are components of the battery, may occur, with the result that heat and gas are generated, and the high-temperature and high-pressure conditions caused by generation of the heat and the gas accelerate the above-mentioned decomposition. Ultimately, the lithium secondary battery may ignite or explode.
The safety-related problem of the lithium secondary battery is even more serious for a middle or large-sized battery module or battery pack having a multi-cell structure. Since a plurality of battery cells is used in the multi-cell battery module or battery pack, abnormal operation of some of the battery cells may cause abnormal operation of the other battery cells, with the result that the battery pack may ignite or explode, which may lead to a large-scale accident.
For this reason, a conventional middle or large-sized battery module or battery pack is provided with a sensing device for measuring the voltage and temperature of battery cells and a battery management system (BMS) for controlling the battery cells based on the measured values.
However, a middle or large-sized battery module or battery pack including such a safety system has a problem in that the capacity of the battery per unit volume is reduced due to the increased space in the battery module or battery pack that is occupied by the safety system.